I am simulating a two cell zone problem with air in one zone, and PCM in the other, connected by a 'coupled' wall. The air zone movement is brought forward by natural convection from a heated surface. The PCM zone is closed, with one side at a temperature lower than the phase change temperaeture so that clear phase change is seen.

My problem is that when gravity is turned OFF, the problem converges rapidly. But as I turn gravity ON, the solution diverges quickly. I need gravity in my problem (Natural convection).